Electrostatic recording method and apparatus for reducing recording area of a stylus



Dec. 16. 1969 M. I. GOLD 3,484,792

ELECTROSTATIC RECORDING METHOD AND APPARATUS FOR REDUCING RECORDING AREA OF A STYLUS F/G. 4 0M A 20 v 2/ 22' FIG. IA /6" PULSE GENERATOR INVENTOR. GENERATOR x MURRAY I. GOLD FIG. -W

ATTORNEY 3,484,792 ELECTROSTATIC RECORDING METHOD AND APPARATUS FOR REDUCING Dec, 16. 1969. M. l. GOLD RECORDING AREA OF A STYLUS Filed Dec. 27, 1965 2 Sheets-Sheet 2 INVENTOR. MURRAY l. GOLD %fi@\ M A TTOR/VE Y United States Patent 3,484,792 ELECTROSTATIC RECORDING METHOD AND APPARATUS FOR REDUCING RECORDING AREA OF A STYLUS Murray I. Gold, Monterey Park, Calif., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Dec. 27, 1965, Ser. No. 516,392 Int. 'Cl. G01d 15/06, 15/08; H04n /76 US. Cl. 346-74 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to electrostatic recording apparatus and more particularly to an improved method and electronic drive apparatus for actuating electrostatic recording stylii.

The electrostatic recording process comprises the steps of printing, inking, and fixing. In the printing step information to be recorded is formed as indicia shaped electrostatic images on a high resistivity surface of a record sheet or web. Subsequently, during the inking step these previously deposited charged areas are rendered visible, i.e., developed, by the application of a finely powdered developing agent or ink which is attracted to the previously charged areas and retained thereover by electrostatic attraction. The third step, which is optional, comprises fixing the toner or developer powder adhering to the latent images on the record sheets. This step may comprise heating a specially coated record sheet and/or subjecting the powdered images thereon to a rolling pressure contact.

The size and shape of an electrostatic image formed on a record medium as a result of the energization of a recording stylus is a function of a combination of parameters including the size and shape of the respective electrodes, the polarity of the voltage applied to the respective stylus and the electric field intensity in the gap between the printing stylus and the backing electrode.

Particularly if the electrostatic recording process is employed where high resolution is a desirable feature, for example, facsimile systems, line or chart recorders, etc. the expansion of the deposited charge pattern due to the stylus dimensions has been a problem. Such expansion of the deposited charge pattern due to the stylus dimension often results in an overlapping of adjacent deposited charge patterns in response to closely spaced actuating pulses. This overlapping of adjacent charge patterns due to the expansion of the deposited charge pattern by the stylus dimension severely limits the resolution of a printing system, i.e., its ability to resolve closely spaced individual actuating pulses as distinct deposited charged areas. This spreading effect due to stylus dimensions may be somewhat minimized by reducing the physical dimensions of a stylus, however, as would be evident this solution is severely limited by practical physical limitations on the minimum size of a recording stylus.

It is accordingly an object of the present invention to provide electrostatic recording methods and apparatus wherein the apparent or effective recording stylus dimension in the direction of recording is less than the corresponding physical dimension.

. It is another object of the present invention to improve resolution in electrostatic recording apparatus.

3,484,792 Patented Dec. 16, 1969 It is another object of the present invention to reduce the effect of expansion of deposited charge pattern attributable to stylus dimensions in an electrostatic recording system.

It is yet another object of the present invention to provide simplified circuitry for controlling the expansion of deposited charge patterns attributable to stylus dimension in electrostatic recording apparatus.

Other objects of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIGS. 1 and 1A show partial block diagrams of electrostatic recording apparatus utilizable in accordance with the principles of the present invention.

FIG. 2 illustrates conventional recording waveforms utilizable in the recording apparatus illustrated in FIG. 1.

FIG. 3 illustrates recording operations of the apparatus illustrated in FIG. 1 in response to the waveforms shown in FIG. 2.

FIG. 4 shows recording waveforms in accordance with the present invention which are utilizable in the recording apparatus illustrated in FIG. 1.

FIG. 5 illustrates recording operations of the apparatus of FIG. 1 in response to the waveforms of FIG. 4.

FIG. 6 is a schematic diagram of a circuit for generating recording waveforms in accordance with the principles of the present invention.

FIG. 7 shows a single waveform produced by the circuit of FIG. 6.

FIG. 8 shows a multiple waveform train produced by the circuit of FIG. 6.

FIG. 1 illustrates an electrostatic facsimile recorder solely for the purpose of illustrating a typical environment in which the present invention may be employed. A drum 10 preferably formed from electrically conductive material is covered with a sheet of insulating material 11. Drum 10 is rotatably supported and adaptive to be driven by a motor 12 which also drives a lead screw 13 through gears 14. An insulating carriage 15 rides on lead screw 13 and carries a recording electrode or stylus 16 which is adapted to be drawn across the surface of the insulating material 11 as drum 10 rotates. A high voltage pulse generator 17 supplies voltage pulses to stylus 16 in response to received video or other recording signals. The voltage pulse applied to stylus 16 causes electrostatic charge to be deposited on insulating material 11 in a pattern determined by the information format applied to pulse generator 17.

In order to develop or make the latent charge patterns visible, a permanent magnet 18 is mounted on carriage 15 adjacent to the surface of drum 11. Magnet 18 carries on its pole proximate record sheet 11 a so-called magnetic brush 19 which comprises a mixture of iron fillings and finely divided pigmented resin particles. The resin particles commonly referred to as toner are electrostatically charged by contact with the iron filings to a single predominate polarity preferably opposite to that of the latent charge deposited by the stylus 16 on the insulating material 11. As the magnetic brush 19 passes over an area of previously deposited latent electrostatic charge, resin particles are attracted to the areas of charge and adhere thereon thus producing a visible image. As described such an electrostatic recorder may, for example, comprise a facsimile recorder in which a remote-conventional facsimile transmitter selectively actuates pulse generator 17 with conventional video and control signals. FIG. 1A illustrates a high speed electrostatic recording apparatus utilizable in accordance with the principles of the present invention. The stylii 16 are equally spaced about the periphery of drum 15 and cooperably juxtapositioned with curved backing electrode 10. As the drum 15' rotates, suitable switching means selectively couples recording signals from pulse generator 17' to the appropriately positioned stylii 16 thus creating a latent image on record 11'. More detailed information relating to the recording process will be given later.

FIG. 2 illustrates a portion of a typical waveform which might be applied to recording stylus 16. Two closely spaced short duration pulses and 21, each having a duration t and a longer pulse 22, having a duration t are shown. The pulses will typically, although not necessarily, be of a negative polarity and have an amplitude in the order of 500 to 1,000 volts. Depending on design configuration and parameters of the recording apparatus, pulses having a magnitude less than a threshold value, for example, in the order of 500 volts, will not be effective in transferring charge from stylus 16 to insulating material 11. These pulses are ordinarily produced by a pulse generator 17 such as a vacuum tube switch, a transistor switch or a gated DC multi-vibrator transformer.

FIG. 3 illustrates the recording or printing effect when the pulses shown in FIG. 2 are applied to a recording stylus 16 which has a width W and a relative velocity V with respect to the insulating material or record sheet 11. As schematically illustrated, the charge pattern deposited by pulse 22 and the charge pattern deposited by pulses 20 and 21 and made visible by attraction of toner particles extends over or covers a length equal to the dimension W of the stylus pulse the distance transversed by the stylus during the time duration of the respective pulse, i.e., W+Vl or W+Vt where V and t are the velocity and the pulse time duration respectively. The desired length of deposited charge pattern is normally proportional to the pulse duration, i.e., only Vt, or V1 Because of the expansion of the size or spreading effect of the deposited charge pattern by the stylus dimension W, the charges deposited by the pulses 20 and 21 are seen to overlap. Thus, the illustrated system is incapable of resolving pulses which are as closely spaced as the illustrated pulses 20 and 21. Resolution can obviously be improved by reducing the dimension W of the stylus 16 or the velocity but there are practical limitations on how far this process can be carried.

FIG. 4 illustrates a train of recording waveforms in accordance with the principles of the present invention. Recording pulses 20 and 21 and 22' are shown to be similar to those in FIG. 2. However, each recording pulse is immediately followed, i.e., upon the trailing edge thereof, by a pulse 40 of the opposite polarity. The length or duration of erase or neutralizing pulse 40 has not been found to be critical and the amplitude may be considerably less than that of recording pulses 20, 21, and 22. For example, the opposite polarity erase pulse 40 may be in the order 100 volts.

The effect of using this neutralizing or erasing waveform 40 immediately after a printing pulse in an electrostatic system is illustrated in FIG. 5. As shown a latent charged area is deposited or created on insulating member 11, as indicated by the minus symbols, in response to pulses 20, 21' and 22. The stylus 16 is shown in its position at the termination or trailing edge of recording pulse 22'. As hereinabove stated, the trailing edge of a recording waveform, for example 22', is in accordance with the invention, followed by an erasing or neutralizing waveform 40. As herein above mentioned, pulse 40 new tralizes or erases all the previously deposited charges which lie beneath stylus 16. In fact, experimental evidence suggests that previously charged areas may be recharged to the opposite polarity, as shown by the plus symbols not withstanding the fact that pulse 40 may have a voltage considerably less than the normal threshold voltage for depositing charge on insulating material 11. Areas of reversed charge, if they exist, will repel rather than attract toner particles 30. Charges which are no longer beneath stylus 16 are, of course, unaffected by pulse 40. The length of the uncovered, and hence unaltered, area of deposited charge is simply v1 or W, which is the desired charge length or pattern corresponding directly to the length of applied pulse 22. Accordingly, recording proceeds as if the actual stylus 16 of dimension W were replaced by a stylus of infinitely small dimension situated at the left hand or trailing edge of the actual stylus 16. The same considerations apply to the recording of pulses 20' and 21' which are now recorded as shown as separate and distinguishable areas of charge having a length and separation proportional to pulse duration, t and pulse separation, respectively.

The waveforms of FIG. 4 can be produced by known forms of circuitry. For example, the trailing edges of normal printing pulses 20', 21', 22' can be used to trigger a one-shot multi-vibrator which couples a reverse polarity pulse to stylus 16. However, a simpler and equally effective circuit embodying another aspect of the present invention is illustrated in FIG. 6. This circuit consists of a pulse generator 61, which will be described first, and a multi-vibrator 60. Q4 is an emitter follower which couples a positive input pulse applied to its base through capacitor C4 to driver transistor Q5. Transistor Q5 conducts in response to this pulse and permits the supply voltage to be applied through the primary of a pulse transformer T1. The transformer output is a voltage of in the order of 1000 volts which appears at terminal 62 for connection to a recording stylus 16. The output voltage can be varied somewhat by series dropping resistors R14 and R16. Transformer T1, like any transformer, will have a certain amount of leakage inductance and distributed capacitance, which are used to advantage in the illustrated circuit. External inductors or capacitors may be added as shown across the output winding, but this is not ordinarily necessary or desirable.

At the end of the input pulse applied transistor Q4, transistor Q5 will cease to conduct and the energy stored in the transformer leakage inductance must be dissipated. Diode CR6 and resistors R12 and R16 constitute a conventional circuit for dissipating this stored energy. However, the value of the resistors is deliberately chosen high enough, so that relatively little damping action takes place. Instead, much of the stored energy is used to charge up the distributed capacitance which then discharges through the transformer. This process is simply an LC resonance phenomenon of the transformer, sometimes referred to as ringing, and such action produces the output voltage reversal or overshoot 40 illustrated in the waveform of FIG. 7. This voltage reversal or overshoot is a complete functional equivalent of pulses 40 of FIG. 4. The amplitude and duration of the voltage reversal is controlled by the transformer parameters, including any externally connected reactances, and resistors R12 and R16.

The maximum output pulse length of pulse generation 61 is, as is known in the art, limited by transformer T1 and longer pulses generally require a physically larger transformer. Accordingly, the illustrated circuit presents a simple and economical way of generating electrostatic recording signals in accordance with the invention, but only if the required pulse length is in the micro-second or milli-second range.

Where longer pulses are required than can conveniently be delivered by transformer T1, use may be made of multivibrator 60 of FIGURE 6. This is an asymmetrical free running multivibrator consisting of transistors Q2, Q3 and associated cross coupling components, and a transistor Q1 which selectively clamps or unclamps the multivibrator. With transistor Ql in its normal or non-conducting state, diodes CR1 and CR8 are forward biased and the baseof Q2 is maintained a few tenths of a volt negatwo with respect to its emitter, by the voltage drop across CR8. In this condition Q2 is cut off and Q3 is conducting. When a positive signal is applied to the base of Q1 the transistor will conduct; the emitter voltage will rise toward the collector voltage. This will cause both diode CR1 and CR8 to be back-biased and therefore become inefiective in controlling the operation of transistor Q2. Transistors Q2 and Q3 will then operate as a free-running multivibrator until the positive input voltage is removed from transistor Q1. The waveform produced at the output terminal 62 will be as shown in FIGURE 8 for the circuit parameters shown in FIGURE 6. The successive recording pulses of FIGURE 8 will ordinarily be so closely spaced in time and therefore so closely spaced physically on insulating material 11 that they will produce a continuous mark when developed by toner particles 30 as shown in FIG. 3. This may occur because the individual areas of charge laid down by stylus 16 are so closely spaced as to exceed the resolution capability of the development system, because the resolution capability of stylus 16 and the associated pulse generator is less than infinite and has been exceeded, or by a combination of these causes. The actual reasons are immaterial since the effect may be reliably reproduced. Accordingly, the visible marks made on insulating material 11 are directly proportional in length and spacing to the duration and timing of the signals applied to transistor Q1, even though the illustrated circuit is only capable of producing pulses of constant length.

In order for the stylus 16 to appear to be of zero width, only the last pulse in each group of recording pulses has to be followed by an overshoot or neutralizing pulse 40 and 40 as shown in FIGS. 4, 7 and 8, respectively. That is in accordance with another aspect of the present invention when multiple pulses are to be recorded as a continuous mark the spreading effect of the stylus dimension may be effectively eliminated by utilizing the neutralizing pulse 40 or 40 as shown in FIGS. 4, 7 and 8, respectively upon the trailing edge of the last pulse in the group. However, as hereinabove stated, the normal pulse emanating from the circuitry of FIG. 6 and illustrated in FIGS. 7 and 8 may be employed to record closely spaced pulses which due to overlapping are recorded as a continuous line.

In an illustrated embodiment of the present invention, a resolution of greater than 100 lines per inch was easily achieved with apparatus similar to that shown in FIG. 1 by employing a stylus having a dimension W of .017 inch and a velocity of 15 inches per second. As may be seen by those skilled in the art this is greatly in excess of the resolution to be expected from the indicated stylus width. These results were obtainable with either single pulses as shown in FIG. 7 or the pulse train shown in FIG. 8.

Insulating material 11 should have sufiiciently high resistivity to retain an electrostatic charge and should be as thin as possible. It may be in the form of a thin sheet of material attached to drum 10 or may be a coating applied to drum 10. A preferred form of insulating material 11 comprises a sheet of paper which has been impregnated to increase its electrical conductivity and which has been coated or impregnated on the surface to be contacted by stylus 16 with a very thin layer of insulating resin. Such papers are described in the patent literature and are commercially available under the name Videograph Paper.

Development of the electrostatic charges formed on the insulating material 11 may be by magnetic brush development as hereinabove explained in conjunction with FIGURE 1, however, cascade development, liquid development, or any of the other known means for developing electrostatic images may likewise be employed. Each of the methods specifically mentioned is used in one or more commercial electrostatic ofiice copying machines and suitable developer materials may be secured from the manufacturers of these machines. All manufacturers supply materials suitable for the development of negative charges to the exclusion of positive charges and some manufacturers also supply material suitable for the development of positive charges to the exclusion of negative charges.

The apparatus of FIGURE 1 is shown for illustrative purposes only since any other machine configuration which provides relative motion between an electrical recording stylus and a sheet of insulating material may equally well be employed. Such machines may be used for facsimile recording, alphanumeric character printing, and many other purposes. A single stylus may be employed or a multiple array of stylii. The stylus dimensions would be chosen in accordance with the contemplated application of the apparatus. The circuits employed in practicing the described method may depart considerably from those described or illustrated as will be apparent to those skilled in the circuit art. Even where the stylus dimension W can be made small enough to provide the desired resolution without resort to the present invention, the present invention will still provide highly desirable benefits in terms of circuit simplicity and economy. As various modifications may be incorporated by those skilled in the art in practicing the disclosed invention, it is applicants intention to be limited only as indicated by the scope of the appended claims.

What is claimed is:

1. In an electrostatic recording apparatus including at least one electrostatic recording stylus adapted for translatory motion relative to an insulating record medium, and additionally including pulse generating means coupled to said stylus for selectively supplying a recording pulse of a first polarity, duration and threshold amplitude sufficient to effect charge transfer from said stylus to said insulating record medium, the improvement comprising circuit means coupled to said stylus for applying an erase pulse having a leading and trailing edge and being opposite in polarity to said first polarity, said leading edge of said erase pulse commencing substantially upon the trailing edge of said recording pulse 2. The improvement defined in claim 1 in which the amplitude of said erase pulse is less than said threshold amplitude.

3. The improvement defined in claim 1 further including means for developing latent electrostatic charges deposited on said insulating record member, said developing means being effective to develop areas charged to a polarity corresponding to the polarity of said recording pulse and wherein said recording and erase pulses are generated sequentially by the same circuit means.

4. In an electrostatic recording apparatus including at least one electrostatic recording stylus adapted for translatory movement relative to an insulating recording medium the improvement comprising pulse generator means coupled to said stylus for providing a sequence of closely spaced repetitive recording pulses in response to a maintained input signal, at least a portion of each of said pulses having a first polarity, duration and amplitude sufiicient to effect transfer of charge from said stylus to said insulating recording member and at least the trailing edge of the last pulse in any sequence of closely spaced pulses being substantially immediately followed by an erase pulse of a second polarity opposite to said first polarity and having a leading and trailing edge.

5. The method of electrostatically recording information comprising the steps of applying a first polarity record pulse to an electrostatic recording stylus during relative translatory motion thereof over a surface of a body of insulating record material, and

applying a second polarity erase pulse having a leading and trailing edge and opposite in polarity to said first pulse to said stylus while part of the charge transferred by said stylus in response to said record pulse is still beneath said stylus, thereby removing from beneath said stylus substantially all charges of said first polarity.

6. In an electrostatic recording apparatus including at least one electrostatic recording stylus adapted for translatory motion relative to an insulating record medium, and additionally including pulse generating means coupled to said stylus for selectively supplying a record pulse of a first polarity, duration, and threshold amplitude sufiicient to effect charge transfer from said stylus to said insulating record medium, the improvement comprising erase pulse means coupled to said stylus for neutralizing charge transferred from said stylus to said record medium, said erase pulse means becoming effective substantially at that point in time coincident with the termination of said recording pulse.

7. In the process of electrostatically recording infor mation wherein a recording stylus moves over a surface 'of an insulating record medium, the method comprising the steps of applying a recording pulse of a polarity and threshold amplitude to said stylus, said amplitude sufiicient to References Cited UNITED STATES PATENTS 2,200,741 5/1940 Gray 34674 3,166,753 1/1965 Ryerson 34674 3,247,517 4/1966 Stone 34674 BERNARD KONICK, Primary Examiner J. F. BREIMAYER, Assistant Examiner U.S. Cl. X.R. 178-6.6 

